Thin oxide coatings are widespread in industry, in particular in optics, as protective coatings or for optical function purposes. Thus, they can be used as protection against corrosion and mechanical damage or for coating the surfaces of optical components and instruments, such as, in particular, lenses, mirrors, prisms, etc. In addition, thin oxide coatings are used to produce high-, medium- and low-refraction optical coatings for increasing or reducing reflection. The main area of application is the production of antireflection coatings on spectacle lenses and on elements for camera lenses, binoculars and optical components for optical measuring instruments and for laser technology, furthermore for the production of coatings having a certain refractive index and/or certain optical absorption properties, for example in interference mirrors, beam dividers, heat filters and diathermic mirrors.
The starting materials for the production of these oxide coatings are known per se. Conventional materials are SiO.sub.2 and a wide range of metal oxides, possibly in combination with one another. Selection is essentially empirical, depending on the desired optical properties and the processing properties. The coatings are produced by the vacuum vapor-deposition method, which is known per se. For an exemplary illustration, German Patent 12 28 489 is cited here, which indicates materials which can be used and processing methods and the problems which occur in this connection.
For the production of high-refraction coatins, i.e. coatings which have an optical refractive index of around 2, e.g., about 1.9 to about 2.4, preferably 2.0 to 2.2, the choice of starting materials which are suitable in principle is limited. Suitable starting materials are essentially the oxides of titanium, zirconium, hafnium and tantalum, and mixed systems thereof. The preferred starting material for high-refraction coatings is titanium dioxide.
However, these materials which are suitable per se have a number of disadvantages, which are evident, in particular, from practical processing.
One aspect here is that these substances have high melting and boiling points, which, in addition, are relatively close to one another. From a practical point of view, however, it is important that the vapor-deposition materials are in the fully-melted state before noticeable deposition begins. Only then is a uniform and adequate deposition rate ensured. This is necessary so that homogeneous and uniformly thick coatings form on the objects to be coated. In the case of the oxides of zirconium and hafnium and in the case of titanium/zirconium mixed oxide systems, however, this is not the case under practical application conditions. Said substances do not melt completely, or at all, under conventional working conditions. They are overall difficult to evaporate, and coatings having thickness variations are obtained.
It is therefore desired to reduce the melting points of the base materials by means of suitable additives. Additives furthermore serve to vary within certain limits and set in a specific manner the refractive index in the resultant coating.
The choice of suitable additives for this purpose is restricted by the requirement for freedom from absorption. The only metal oxides which are therefore suitable as corresponding additives are those which have no absorption in the visible spectral region and into the near UV wavelength range (up to about 320 nm).
A further problem is the following:
Although said oxides, as starting material, have only low absorption, or none at all, in the visible wavelength range, which is a prerequisite for the corresponding optical applications, the production of thin coatings by vacuum vapor deposition using these materials nevertheless results, without special precautions, in coatings of high absorption in the visible regions. This is particularly true of titanium dioxide, which is regarded as the starting material of choice for high-refraction coatings. This effect is attributed to loss of oxygen during high-vacuum vapour deposition and the deposition of titanium oxide coatings which are substoichiometric with respect to the oxygen content.
This problem can be circumvented by carrying out the vapor deposition under a vacuum with a certain oxygen residual pressure (about 10.sup.-4 -10.sup.-5 mbar), i.e. with an oxidizing character, and/or subjecting the resultant coatings to subsequent conditioning in oxygen or air. According to the abovementioned German Patent 12 28 489, this technique is particularly advantageous for adding elements or oxides from the rare earths to the materials to be evaporated.
Although the abovementioned problems can be solved by a suitable choice of additives or the selection of appropriate mixtures of materials, the use of mixed systems is not preferred per se in the vacuum vapor-deposition method. The reason is that mixed systems generally evaporate incongruently, i.e. they change their composition during the evaporation process and the composition of the deposited coatings also changes correspondingly. This can normally only be avoided if the mixed systems are discrete chemical compounds which evaporate without material change and can be recondensed.